It's quite common to see web pages that show a solid color in place of
images, while resources are being loaded. The color is usually
averaged from the original image, as to give it a more "consistent"
feeling while the page loads.

A simple way to do this from the command line is by using ImageMagick
to resize the image to 1x1 size, then get the color of that one pixel:

At BKNO3 we use Docker and docker-compose for development, in order
to make sure every developer can easily and reliably set up a
development environment locally, even if they're not familiar with the
project itself (we have several components, with different developers
working on each one).

The same docker containers are used to run continuous integration (on
CircleCI) as well, in order to reduce the differences between the
dev/ci/prod environments.

A big pain in doing so, however, was the time spent rebuilding the
whole docker image every time the test suite was run, even if nothing
changed ...